Several substances have already been extracted from medicinal leeches (hirudo medicinalis) such as polypeptides that act as proteinase inhibitors and which have, in part, an antithrombin action. In the literature that deals with these substances, a distinction is drawn between the so-called eglines and hirudins. Two eglines are described in DE-PS No. 28 08 396. The extraction of crude hirudin is described in Die Pharmazie, Number 36, 1981, pages 653-660 and in Methods in Enzymology, Volume 45, 1976, pages 669-678. The complete amino acid sequence is known from FEBS 1104 (Federation of European Biochemical Societies, Volume 165, 1984, pages 180-184).